1. Field of the Invention
The present invention relates to a cold-rolled ultra low carbon steel sheet having excellent press-forming properties.
2. Description of the Prior Art
The improvement in the press-forming properties of a steel sheet depends upon the annealing process subsequent to the rolling operation. In this connection, many studies have thus far been made that advance various proposals with regard to the combination of the steel composition and the annealing process. Steel compositions that can be imparted with a high press-forming property during annealing within a short time period by resorting to quick heating and quick cooling cycles, as in continuous annealing, are advantageous from the standpoint of productivity. Examples of such steel include Ti-added ultra low carbon steel and ultra low carbon alumi-killed steel.
Ti-added ultra low carbon steel has been regarded as the sole material which withstands severe press-forming operations. However, production of this steel requires the addition of Ti in a large quantity and the use of high vacuum-degassing techniques, such as the DH method, in view of the amount of added Ti. In addition, annealing at a higher temperature over a longer time period than that required for ordinary rimmed steel is required, thereby increasing the cost of manufacture.
On the other hand, ultra low carbon alumi-killed steel has been proposed as a steel material which is cheaper than the Ti-added steel but which still has relatively good press-forming properties (Japanese Patent Publication Nos. 53-22052 and 53-39850). However, it suffers from an inherent drawback in that the steel sheet must be wound up in a high temperature region in order to accelerate the precipitation of AlN, even though this leads to the production of an increased amount of scale. This scale imposes a greater burden in the subsequent acid-washing stage and causes deterioration of the surface of the steel plate. The quality of the opposite end portions of the coils which are taken up at lower temperatures is also poor. In addition, ultra low carbon alumi-killed steel requires annealing at higher temperatures than rimmed steel and, notwithstanding its small carbon content of about 0.01%, needs complete overaging treatment as with ordinary steel to eliminate degradation of press-forming properties by aging.
In order to provide overaging, which is required for enhancing the forming properties as mentioned above, annealing has to be provided with a heat cycle that is particularly designed for this purpose. Line I in the graph of FIG. 1 shows a typical heat cycle in a conventional annealing process in which, subsequent to heating at a high temperature level (a), a predetermined temperature (b) is maintained for a long time period to ensure sufficient overaging. Curve II is a heat cycle of a recently developed complete overaging process, involving a complicated operation of repeat heating and cooling that is disadvantageous in view of the burden on equipment and the increase in operational steps.